This invention relates generally to foamed polypropylene copolymers, their uses and to the methods of their production.
Among the three most versatile commodity plastics, which are polyethylene (PE), polystyrene (PS) and polypropylene (PP), polypropylene is considered to possess the most favorable properties profile of the three for a variety of applications. These applications include, for example, oriented and non-oriented films, textile fibers, nonwovens and a variety of injection molded parts. Comparing the properties, it is well known that polypropylene has a higher modulus and heat deflection temperature (HDT) than polyethylene. The higher the modulus and HDT, the more suited the polymer is for durable applications in the appliance and automotive segments. Additionally, because polypropylene is nonpolar, it resists degradation by common solvents, such as acids and alkalis. Compared to polystyrene, polypropylene is preferred in applications requiring good organoleptic performance, high barrier properties and the living hinge property. Finally, polypropylene blends well with a variety of other polymers, and in impact-modified form occupies a dominant position in the automotive industry in the areas of bumpers, side panels, floor mats, dashboards and instrument panels.
However, there exist some polymer applications where polypropylene is not the preferred plastic of choice. Examples of such polymer application areas include thermoforming and foaming. Foamed polymers find usage in automotive, marine, appliance and packaging applications because of their good insulating and structural properties at low added weight. Thermoforming is a popular fabricating mode that competes favorably with injection molding in the making of thin-walled containers. Polypropylene""s deficiencies in foaming and thermoforming are believed to be related to its generally poor melt strength and rapid melt viscosity drop, poor sheet sag and comparatively slow crystallization kinetics. For example, to successfully foam an article formed from a polyolefin, it is desirable that the polyolefin selected for foaming possess high melt strength. With high melt strength, the bubble growth rate within the polyolefin can be controlled without premature bursting. Controlling bubble growth rate is also important for ensuring a uniform distribution of cell sizes, which leads to greater product uniformity. Additionally, broader polymer processing temperature windows are desirable so that when the polymer is used in an article forming process, the temperature variances along the process line are less disruptive to the fabrication of a quality product.
It has been discovered that propylene polymers prepared in multistage reactors that exhibit broad molecular weight distribution and broadened melting ranges offer the capability to provide better control during foam fabrication than do traditional propylene polymers. The final product is anticipated to be a better quality foam. The present invention is directed toward such a foamed polypropylene having improved characteristics such as stretchability, tensile strength, and broader processing temperature windows.
Specifically, this invention relates to a crystalline propylene polymer foam comprising (i) from 10 to 90 weight percent isotactic crystalline propylene homopolymer having a molecular weight distribution (Mw/Mn) less than 3.0 and (ii) from 90 to 10 weight percent crystalline propylene copolymer including propylene units and comonomer units, wherein the crystalline propylene copolymer has a molecular weight distribution less than 3.0, and wherein the comonomer weight percent based on the total crystalline propylene polymer weight is in a range from 0.1 to 5. The crystalline propylene polymer has a molecular weight distribution (Mw/Mn) in a range from 2.1 to 10. This polymer, when used in state-of-the-art foam fabricating equipment, is anticipated to yield foams with low product densities of less than 0.3 g/cm3.
In another embodiment, a crystalline propylene polymer foam is provided which comprises (i) from 10 to 90 weight percent crystalline propylene homopolymer having a molecular weight distribution less than 3.0 and a first molecular weight in the molecular weight range from 25,000 Mw to 300,000 Mw and (ii) from 90 to 10 weight percent crystalline propylene copolymer including propylene units and comonomer units. The crystalline propylene copolymer has a molecular weight distribution less than 3.0 and a second molecular weight greater than the first molecular weight. The crystalline propylene polymer has a molecular weight distribution (Mw/Mn) in a range from 2.1 to 10. The comonomer weight percent, based on total crystalline propylene polymer weight, is in a range from 0.1 to 5. Foams with densities down to less than 0.3 g/cm3 are anticipated on state-of-the-art foam fabricating equipment.
In yet another embodiment, a crystalline propylene polymer foam is provided which comprises (i) from 30 to 70 weight percent crystalline highly isotactic propylene homopolymer having a molecular weight distribution less than 2.5 and a first molecular weight in a range from 25,000 Mw to less than 300,000 Mw; and (ii) from 70 to 30 weight percent crystalline propylene copolymer including propylene units and comonomer units. The crystalline propylene copolymer has a molecular weight distribution less than 2.5 and a second molecular weight greater than the first molecular weight. The comonomer weight percent, based on total crystalline propylene polymer weight, is in a range from 0.8 to 2. The comonomer may be selected from a group consisting of ethylene, and alpha olefins, such as 1-butene, 1-pentene, 1-hexene, and 1-octene or a combination thereof. The crystalline propylene copolymer has a molecular weight distribution (Mw/Mn) in a range from 2.5 to 7.0, a weight average molecular weight in a range from 200,000 to 400,000 and a melt flow rate in a range from 0.5 dg/min to 20.0 dg/min. From such polymers, foams with densities down to less than 0.3 g/cm3 are anticipated on state-of-the-art equipment
In yet another embodiment, a process for preparing a foamed crystalline polypropylene polymer is provided. The process steps comprise (a) polymerizing propylene in a first stage, (b) copolymerizing propylene and a comonomer in a second stage in the presence of the product of the first stage, (c) recovering crystalline propylene polymer comprising from 0.05 to 15 weight percent comonomer units based on the total weight of the polymer and (d) processing the crystalline polypropylene polymer so that when the crystalline polypropylene polymer is contacted with a foaming agent, a foamed crystalline polypropylene polymer is formed. The polymerization steps (a) and (b) are desirably conducted in the presence of a metallocene catalyst system comprising at least two different metallocene catalyst components.